1. Field of the Invention
The present invention relates to a method for manufacturing a microlens that can easily form a microlens having a smooth shape, in which a maximum thickness position is different from a gravity center position.
2. Description of the Related Art
A solid-state image sensor, such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) sensor, which converts the subject light into optical signals in order to record images, is provided in a main body of a digital camera that has been widely used in recent years. In general, the solid-state image sensors comprise light receiving elements that receive and convert the subject light into photoelectric signals, color filters formed on the light receiving elements, and microlenses that improve light collection efficiency in the light receiving elements.
The microlens is very useful in terms of improving light collection efficiency in the light receiving element. However, since the microlens is provided, noise or shading is considerably generated on occasion and therefore becomes problematic. Specifically, when oblique light is thrown on the microlens, light collection is insufficient in the light receiving element as compared to when light is thrown on the microlens in a vertical direction. As a result, difference in light collection occurs between pixels, which causes difference in signal output, and noise is occasionally generated due to output variation. For example, difference in signal output is caused by the oblique light at a central portion and a peripheral portion thereof in each of the pixels. In this case, there is a possibility that the signal output in the peripheral portion of each pixel is smaller than that in the central portion of each pixel. This difference in signal output occasionally causes shading on a screen on which images are displayed.
There has been known an image sensor in which a plurality of light receiving elements share a signal reading circuit for the purpose of high integration, that is, so-called an image sensor having plural pixel sharing structure. However, due to its structural restriction, a central position of the microlens occasionally does not correspond to a central position of the light receiving element in a plan view in the image sensor. For this reason, problems such as noise and shading are caused.
A microlens having a rotationally asymmetrical shape, which is different from the conventional microlens having a rotationally symmetrical shape, has been proposed to solve the above-mentioned problems. That is, an attempt that the microlens having a rotationally asymmetrical shape is used to obtain uniform light collection in the light receiving element and to suppress the problems such as noise and shading.
A method for forming a microlens has been proposed in Japanese Patent Application Laid-Open (JP-A) No. 2002-76315. According to the method, lattice patterns are formed to be asymmetry to a center of a unit cell, a material of the lens is applied and then patterned, and a thermal flow is performed, in order to form a microlens having a rotationally asymmetrical shape. However, according to JP-A No. 2002-76315, the microlens is formed using the thermal flow. For this reason, it is difficult to accurately form a lens having a desired shape.
Meanwhile, a method for forming a microlens has been proposed in JP-A No. 2005-258349. In the method, a double exposure is performed using a gray-scale mask and a corrective aperture mask to form a microlens having a desired shape. However, a rotationally asymmetrical microlens has not been described in JP-A No. 2005-258349.